Noise, as an unwanted side effect of industrial development, causes gradually more damages. A variety of measures are taken to prevent noise. In an effort to prevent the noise, research on the development of new sound-absorbing materials capable of arresting, absorbing or insulating sound has been conducted in various ways.
Representative industrial sectors requiring sound-absorbing materials include electric appliances such as air conditioner, refrigerator, washing machine, lawn mower, etc., transportation such as automobile, ship, airplane, etc., construction materials such as wall material, flooring material, etc., and so forth. The sound-absorbing material is required in other various industrial fields, too. In general, the sound-absorbing materials used in industries require, in addition to good sound-absorbing property, lightweightness, flame retardancy, heat resistance and heat-insulating property, depending on particular applications. Especially, flame retardancy and heat resistance may be further required for sound-absorbing materials used in engines, exhaust systems, etc. maintained at high temperatures of 200° C. or above. At present, aramid fibers are gaining attentions for sound-absorbing materials having superior heat resistance.
In addition, in order to provide functionalities such as flame retardancy, water repellency, etc. to a sound-absorbing material, many sound-absorbing materials wherein a nonwoven fabric containing aramid fibers and a functional skin material are laminated on each other have been developed.
For example, Korean Patent Application Publication No. 2007-0033310 discloses a flame-retardant sound-absorbing material wherein a nonwoven fabric layer in which a heat-resistant short aramid fibers and short thermoplastic polyester fibers are bridged and a skin material layer formed of a wetlaid nonwoven fabric consisting of short aramid fibers are laminated with each other.
Japanese Patent Application Publication No. 2007-0039826 discloses a water-repellent sound-absorbing material wherein a nonwoven fabric layer of a heat-resistant short aramid fiber or a blend of a short aramid fiber and a short thermoplastic polyester fiber and a skin material layer treated with a water repellent are laminated with each other.
Japanese Patent Application Publication No. 2007-0138953 discloses a heat-resistant sound-absorbing material wherein a nonwoven fabric layer consisting of a heat resistance aramid fiber and a skin material layer formed of a fiber sheet containing a heat resistance aramid fiber are laminated with each other.
Since the sound-absorbing materials described above have a structure wherein a skin material layer is laminated on one side of a nonwoven fabric to provide functionalities such as flame retardancy, water repellency, etc., a hot pressing process for integrating the nonwoven fabric layer and the skin material layer is necessary. Consequently, the overall process is complicated and troublesome and a flame retardant, water repellent, etc. included as additives may lead to production of toxic gases as a result of combustion during the hot pressing process. In addition, deformation of the internal structure of the nonwoven fabric that may occur during the hot pressing process can lead to deterioration of sound-absorbing property.